Documentation of controlled_gate() should also mention anti-control capability

[araza6]

http://qutip.org/docs/3.1.0/apidoc/functions.html#qutip.qip.gates.controlled_gate The documentation of controlled_gate() in the link above only mentions the control operations that one can do. However, this can also implement anti-control operations if control_value is set to 0.

I think it should be documented too for ease of use. What do you all think?